The present invention relates to low voltage metal oxide varistors, and in particular to free-standing, thick-film varistors fabricated by screen printing.
In general, a metal oxide varistor comprises a zinc oxide (ZnO) based ceramic semiconductor device with a highly nonlinear current-voltage relationship which may be represented by the equation I=(V/C).sup..alpha., where V is the voltage between two points separated by the varistor material, I is the current flowing between the points, C is a constant, and .alpha. is a measure of device nonlinearity and is a number greater than 1. If .alpha.=1, the device exhibits ohmic properties. For values of .alpha. greater than 1 (typically 20-50 or more for ZnO based varistors), the voltage-current characteristics are similar to those exhibited by back-to-back connected Zener diodes but with much greater voltage, current, and energy-handling capabilities. Thus, if the voltage applied to the varistor is less than the varistor breakdown voltage, only a small leakage current will flow between the electrodes and the device is essentially an insulator having a resistance of many megohms. However, if the applied voltage is greater than the varistor breakdown voltage, the varistor resistance drops to an extremely low value (tenths of an ohm) permitting large currents to flow through the varistor. Under varistor breakdown conditions, the current through the varistor varies greatly for small changes in applied voltage so that the voltage across the varistor is effectively limited to a narrow range of values. The voltage limiting or clamping action is enhanced at higher values of .alpha..
Metal oxide varistors have been widely employed for protecting electrical equipment from voltage transients on AC power lines created by switching of electrical apparatus or lightning storms. Such applications require the use of varistors having breakdown voltages slightly greater than the maximum input voltage of the system to be protected. Thus, for example, a typical system powered from 120 volt AC power mains would require the use of a varistor having a breakdown voltage somewhat greater than 120 volts.
In some applications, however, varistors with much lower breakdown voltages are required. An exemplary application of varistors having breakdown voltages of 50 volts or less is in multiplexing of display cells in large area liquid crystal displays of the type described in U.S. Pat. No. 4,233,603 issued on Nov. 11, 1980 to D. E. Castleberry and which is assigned to the same assignee as the present invention.
One way to fabricate low voltage varistors is to mechanically reduce the thickness of the varistor material by grinding or abraiding, for example. This method is not satisfactory for commercial production since the varistor breakdown voltage may be approximately 6 volts/0.001 inch depending on the formulation of the varistor material. Thus, to produce a varistor having a breakdown voltage of 50 volts, it is necessary to grind or otherwise reduce the thickness of the varistor material to approximately 8 thousandths of an inch. Not only is it difficult to produce varistors of such dimensions by mechanical means, but it is also uneconomical to do so. Moreover, this method does not consistently produce varistors having the desired electrical characteristics. Similarly, conventional methods of fabricating varistors are not readily adaptable to produce thin varistors. Typically such methods require cold pressing the varistor powder prior to firing it. It is difficult to press varistor powder to a thickness of 8-10 thousandths of an inch.
Another approach to fabricating thin varistors, known as thick-film varistors, is exemplified by U.S. Pat. No. 3,725,836 issued to Wada et al. In accordance with this method, the varistor is manufactured by screen printing on an insulating substrate a composite made up of pulverized sintered varistor material, a glass binder and a suitable carrier. The printed composite is sintered at a relatively low temperature of between 600.degree. C.-1000.degree. C. During sintering, the binder evaporates and the glass melts, binding the varistor material particles together and to the insulating substrate. Because the insulating substrate is thus firmly bonded to the varistor, electrodes must be affixed adjacent to each other on the free side of the varistor material. Not as conveniently, the electrodes may be affixed to opposite sides of the varistor, if one of the electrodes is bonded to the insulating substrate prior to screen printing and firing of the varistor composite. In this configuration, one of the electrodes is disposed between the insulating substrate and the body of the varistor. Varistors produced in accordance with the afore-described method are known as "reconstituted" varistors.
The production of screen-printed, reconstituted varistors is not commercially economical since it is first necessary to fabricate a varistor material which must be crushed and pulverized prior to its use in screen printing. The process of "reconstituting" the varistor also requires an additional sintering step. If electrodes are to be affixed to the varistor simultaneously with the sintering of the printed varistor, the electrodes must be of a noble metal alloy, such as platinum-palladium-gold, to withstand the corrosive effects of high temperature bismuth which is typically included in convetional varistor compositions.
One disadvantage associated with screen printed reconstituted varistors is that the varistor material bonds tenaciously to the insulating substrate, leaving only one exposed varistor surface for conveniently attaching the electrodes. This limits the range of usable varistor configurations. Even more troublesome is the lack of reproducibility of varistor properties due to the inhomogenity of the zinc oxide grains in the pulverized varistor particles employed in the reconstituted device. The pulverized varistor particles are in effect small varistor devices, each made up of varying numbers of smaller ZnO grains separated at the boundaries by insulating materials. Since varistor material properties, such as breakdown voltage, are related to ZnO grain size, grain boundary, and thickness of the material between the electrodes, the presence of more ZnO grains (and hence grain boundaries) in one region of the varistor results in the occurrence of nonuniform properties in a single varistor. This lack of grain homogeneity may be, in part, attributable to the relatively low temperature (600.degree. C.-1000.degree. C.) at which the reconstituted varistor is sintered. The mixture is not heated to a sufficiently high temperature to homogenize ZnO grain distribution.
The present invention provides an economical method for fabricating free-standing, thick film varistors employing the screen printing process. The thick film varistors manufactured in accordance with this process are readily separable from the smooth insulating substrate on which they are fabricated, thus permitting electrodes to be easily attached to both sides of the varistor. Since unsintered varistor materials employed in the present invention comprise metal oxide particles as small as 10 millionths of an inch, the resulting varistors have good grain homogeneity and hence exhibit superiorly uniform electrical properties. In accordance with the method of the invention, free-standing, thick film varistors having a surface area in excess of 2.times.2 inches and a thickness of 5 thousandths of an inch have been produced.